Mechanical defibration of wood can be implemented either by grinding or refining. Both methods are based on kneading wood raw material by means of pressure pulses and mechanical separation of fibers from each other. The idea behind the processing is to prepare the wood raw material so that the subsequent mechanical separation of fibers from each other would produce pulp suitable for papermaking, not only wood fibers separated from each other. In the grinding process to which this invention relates, the above-described series of actions is implemented by pressing logs of wood in transverse direction against a rotating cylindrical grinder stone, thus keeping the longitudinal direction of the logs of wood in parallel with the axis of the grinder stone. Grinding segments are attached on the surface of the grinder stone, said segments being composed of wear-resistant grinding grits. The grinding grains in the segments typically form an irregular three-dimensional defibration surface. In the direction of the periphery of the surface, the difference in height due to the random location of the grinding grits produces pressure pulses on the wood raw material. Pressure pulses cause deformations and generation of heat in the wood raw material and as a result of this the wood material becomes softer. The friction between the grinding grits and the wood loosens fibers from the surface of the wood raw material. The greatest drawback of these mechanical defibration methods is their high energy consumption due to the extensive generation of heat. Another weakness is the fact that the properties of the grinding surface, such as the distance between the grinding grits cannot be controlled precisely in said three-dimensional structure. Thirdly, in such a structure all grinding grits have similar characteristics, wherein it is not possible to affect the pressure pulses produced by the grits and the loosening of the fibers independently of each other. Examples of such grinder stones for defibration of wood are disclosed in the U.S. Pat. No. 2,769,286 and in the Finnish patent 68268, whose counterpart is inter alia Canadian patent 1267293.
The U.S. Pat. No. 3,153,511 discloses a device whose defibration surface contains protrusions of a predetermined size at certain intervals. The device may be a rotating cylindrical element in which the grinding surface is composed of sectors positioned successively in the rotating direction and separated from each other by means of spacers. The manufacture of the grinding surface is not discussed in this publication and only metal or abrasion resistant plastic are mentioned as manufacturing materials of the tool. The test results of the device are discussed in the article: Atack, D. and May, W. D., 1962, Mechanical pulping studies with a model steel wheel, Pulp and Paper Magazine of Canada, Vol. 63:1, T10-T20. According to these results the device does not work because the grinding surface was composed of completely smooth metal protuberances that produce only treatment that heats the wood.
Publication FI-98148, whose counterpart is inter alia U.S. Pat. No. 6,241,169, discloses a method using energy more efficiently than conventional methods used in the industry, because the method utilizes as large an amount of the energy as possible for breaking the wood raw material structure before it changes into thermal energy. This method utilizes the wavelike shape of the defibration surface and the regular defibration surface in the peripheral direction. The manufacture of such a defibration surface used in industrial scale is challenging for example due to the precise working or formation of the wavelike metal surface.